Prior to the present invention, Bruce A. Ashby et al, U.S. Pat. No. 4,278,804, disclosed various ultraviolet light-absorbing agents which were used in silicone harcoats some of which are included by the formula, ##STR1## where Y is H or OH, Z is H, OH, OQ or OW, where at least one Z is OH in the ortho position to the carbonyl if Y is H; Q is --CH.sub.2 (CH.sub.2).sub.n Si(R.sup.1).sub.x (OR).sub.y ; and W is C.sub.m H.sub.2m+1 ; x=0, 1 or 2, y=1, 2 or 3, x+y=3, R=alkyl or alkanoyl having 1 to 6 carbon atoms, R.sup.1 =alkyl having 1 to 6 carbon atoms, n=0, 1 or 2, and m=1 to 18.
Some of the silicone hardcoat formulations to which the compositions of formula (1) were employed as ultraviolet light-absorbing agents are shown by Frye, U.S. Pat. No. 4,277,287, assigned to the same assignee as the present invention. These silicone hardcoat formulations comprise the hydrolysis product of an aqueous dispersion of colloidal silica and an organotrialkoxysilane of the formula, EQU R.sup.2 Si(OR.sup.3).sub.3 ( 2)
where R.sup.2 is alkyl having 1-3 carbon atoms, or aryl having C.sub.(6-13) carbon atoms, and R.sup.3 is C.sub.(1-8) alkyl.
Although valuable results were achieved by using the ultraviolet light-absorbing agents of formula (1) in silicone hardcoat formulations resulting from the employment of organotrialkoxysilane of formula (2), the "age-in" time required for cohydrolyzing the UV light-absorbing agents under ambient conditions into the silicone hardcoat formulation, often required 4-6 weeks. As a result, the weight percent of the ultraviolet light-absorbing agent actually equilibrated into the silicone hardcoat formulation was often insufficient to impart a satisfactory degree of weatherability resistance and thermoformability to thermoplastic substrates, such as polycarbonate sheets treated with such UV-stabilized silicone hardcoat formulation and thereafter cured.
As used hereinafter, the term "age-in" means the time required for ultraviolet light absorbing agent utilized in the practice of the invention, or as shown by formula (1) to be sufficiently equilibrated into the silicone hardcoat formulation to produce a desirable coating. For example, an ultraviolet light-absorbing agent of formula (1) would be sufficiently aged-in, or equilibrated into the silicon hardcoat formulation, if the UV-stabilized silicone hardcoat formulation resulting from the incorporation of the ultraviolet light-absorbing agent after the age-in period could be coated onto a transparent thermoplastic test panel, for example, a polycarbonate test panel, to produce a crack-free clear film after a 30 minute air dry and a cure for 90 minutes at 135.degree. C.
Improved age-in performance of ultraviolet light-absorbing agents into silicone hardcoat formulations has been found if ultraviolet light-absorbing agents having the formula, ##STR2## are used, where Y and Z are as previously defined, Q' has the formula, EQU --CH.sub.2 (CH.sub.2).sub.n Si(OCH.sub.3).sub.3
and n is as previously defined in formula (1).
It also has been found that ultraviolet light-absorbing agent of formula (3), having trimethoxy substitution on the silicon atom can be cohydrolyzed at ambient temperatures into silicon hardcoat formulation within a week. In addition to having superior age-in time, the weight percent of the cohydrolyzed trimethoxysilyl substituted benzophenone can exceed 25% by weight of the total solids in the silicone hardcoat formulation. Accordingly, the weatherability and thermoformability of cured silicone hardcoats on thermoplastic substrates containing ultraviolet light absorbing agents of formula (3) can be substantially enhanced. However, experience has also shown that the employment of ultraviolet light-absorbing agents of formula (3) having trimethoxysilylalkyl substitution often result in silicone hardcoat formulations which prematurely gel, and therefore are rendered useless for silicone hardcoat applications because the ultraviolet light stabilized silicone hardcoat formulation does not have sufficient shelf life.
Prior to the present invention, methods available for making ultraviolet light-absorbing agents of formula (3), substituted with trimethoxysilylalkyl groups, as shown above, were generally limited to procedures utilizing trimethoxysilane which is known for its extreme toxicity. For example, trimethoxysilane can be added to an allyloxy derivative of hydroxybenzophenone via a hydrosilation reaction to produce an ultraviolet light-absorbing agent within the scope of formula (3). However, this platinum catalyzed addition of such silicon hydride would not be feasible in large scale production due to the toxicity problem previously discussed. It would therefore be desirable to develop a method for producing an ultraviolet light-absorbing agent for silicone hardcoat formulations which would have a reduced age-in time in the silicone hardcoat formulation without resulting in premature gelation and which would be based on procedures which could be utilized in large scale operations without any danger of toxicity.
The present invention is based on my discovery that ultraviolet light-absorbing agents of the formula ##STR3## where Q" is EQU --CH.sub.2 (CH.sub.2).sub.n Si(OCH.sub.3).sub.3-a (OR).sub.a,
Z, Y, R and n are as previously defined and "a" has an average value of from about 0.5 to about 2.5 inclusive, camn be made readily to provide UV stabilized silicone hardcoat formulations having improved age-in time without suffering from premature gelation time during the shelf period. The methoxy-alkoxysilylalkyl-substituted benzophenones of formula (4), where R is preferably C.sub.2 H.sub.5, do not require the use of trimethoxysilane, but can be readily synthesized by equilibrating an ethoxysilyl alkyl-substituted benzophenone within the scope of formula (1) with methanol in the presence of an effective amount of an acid catalyst.